1. Field of the Invention
The invention relates to a Friend Identification (FI) system. More specifically the invention relates to the use of a sequential probability ratio test (SPRT) in making decisions in a Friend Identification system.
2. Description of the Prior Art
A Friend Identification system usually includes a device known as a `Reply Evaluator` or `Decider` that makes `Friend-Accept` decisions in accordance with a specific procedure or algorithm after examining the replies received in response to a sequence of interrogations directed toward a detected but unknown target. If certain specified criteria are satisfied, the device makes an "Accept" (A) decision, and if they are not satisfied it makes a "Not Acceptable" (A) decision.
In a cryptosecure interrogation-reply friend identification system, the interrogations are selected at random, and the correct reply to each interrogation is specified by the cryptosystem whose key setting is possessed only by friends. A friend's chance of providing the correct reply to any particular interrogation depends on the reliability of the equipment involved and the transmission medium. The friend's chance of providing a correct reply may be reduced by purposeful enemy interference or jamming, but the intensity and/or complexity of jamming required to render our friend identification system ineffective depends partly upon how efficiently we use the data that we can collect.
We are concerned here only with a cryptosecure friend-identification system in which a very large number of interrogations are possible, but in which the number of possible replies to each interrogation is very small. Thus, it is not possible for the reply evaluator to make a Friend-Accept (A) decision on the basis of a single acceptable (correct) reply. It must instead examine (observe) a sequence of replies elicited by a sequence of interrogations. Each sequence of replies elicited as the interrogator's antenna scans past (or `looks` at) an unknown target corresponds to a sequence of observation in a sample of data to be evaluated, where the sample may be drawn at random from an `acceptable` lot, the acceptable lot consisting of replies from a friend, or from an `unacceptable` lot, and the unacceptable lot consisting of replies from a non-friend.
If the A and A decisions could always be made on the basis of a single scan past (look at) an unknown (target), then a Sequential Probability Ratio Test as defined in
A. wald, "Sequential Analysis of Statistical Data: Theory" New York, Columbia University Statistical Group Report 75 (AMP Report 30.1) Sept. 1943 (Ref. 1) PA0 A. wald, "Sequential Analysis", New York, John Wiley and Sons, Inc. 1947 (Ref. 2) PA0 W. b. bishop and W. K. Gardner, "On the Sequential Probability Ratio Test in Friend Identification System", NRL (Naval Research Lab.) report 7916, Aug. 19, 1975. (Ref. 3)
could be used with little modification to make these decisions. The `Sequential Observer` decision device in the SAGE A11 Target Evaluator (SATE) does, in fact, use a variation of the Sequential Probability Ratio Test to make its decisions. The Sequential Observer gives each reply received a particular weight in accordance with the number of additional replies received having phase relationships such that they might have come from an enemy who is trying to appear as a friend, i.e., to `spoof` the Friend Identification system. Since these so-called `spoofing` signals could very well come from nearby friends, the Sequential Observer's method of making Friend-Accept (A) decisions is open to serious question (6).
The Sequential Observer bases decisions upon data collected during a single look at the unknown (target) whenever such data satisifies specific requirements for either an A or an A decision. But if the data from look does not call for a decision, the Sequential Observer collects another sequence of data from an additional look, and this process may be repeated several times. No use is made by the SATE of information collected during any look if it is not adequate to warrant making either an A or an A decision having a specified confidence level except that if neither decision is made after some predetermined number of looks, then the operator may make an A decision, and the operator may base his decision on the results obtained during several looks. In other words, the SATE A11 Target Evaluator does not correlate data obtained over several looks at the same target. Hence, some potentially useful data may be thrown away.
The present invention recognizes the fact that a friend's antenna may be in such a position (e.g., hidden by a wing) that no replies, or a very few replies, can be obtained during one or more looks. This requires having the capability of making an A decision on the basis of a single look whenever replies can be elicited from a friend. However, the invention also recognizes that there is no need to throw away data from a look because it is not quite good enough to warrant making an A decision. The data can be used along with data from succeeding looks to make a final A or A decision.